1. Field of the Invention
This invention relates to anodes that are useful for cathodic protection against corrosion. These anodes are particularly intended for use in the protection against corrosion of lock gates in canals, elevated water-storage tanks, and of underground pipes and pipelines, and are of potential use with submarines, ships, and off-shore structures such as oil well drilling platforms.
2. Description of the Prior Art
the corrosion of metallic structures that are buried in the soil or that are immersed in water can be reduced or stopped by cathodic electroprotection. This is accomplished by applying a small electric current from an outside source to the structure that is subject to corrosion. One ampere of applied current will stop corrosion on 500 square feet of uncoated steel, for example.
In cathodic electroprotection, the current is applied through an anode. The anode is connected electrically to a source of positive potential, and is disposed in the soil or sea water so that it is not directly electrically connected to the metallic structure that is to be protected, although it may be mounted on that structure. The metallic structure in turn is connected to a negative source of potential. The anode thus is the positive terminal in the corrosion battery, and the structure is the negative terminal.
In cathodic electroprotection, when an electrical circuit is established between the anode and the structure that is to be protected, through an electrolyte, the resulting current flows from the anode to the structure. This flowing current maintains the structure cathodic at the expense of the anode. The anode is progressively dissolved or sacrificed, so that corrosion of the structure is reduced or prevented.
For the past several years, silicon-iron and graphite have been in widespread use in the cathodic electroprotection anode. These materials are brittle, and have consumption rates on the order of about 1 pound per ampere year. That is, if one ampere of current is passed through the anode for one year, about 1 pound of the anode will be consumed. Consequently, when these materials are used, large anodes are required. Such large anodes are vulnerable to damage from debris and ice, and are also prone to field installation problems.
Other electrically conducting materials have been proposed and used as anodes in cathodic electroprotection systems. These include platinum and platinum-coated titanium or niobium. While reference is made herein to titanium and niobium, generally even the commercially available materials that are considered to be substantially pure titanium and niobium metals are alloyed at least with minor amounts of other materials, and it should be understood that all references herein to these two metals in particular refer to the commercially available 98 to 100 percent pure metals. These substrate materials are essentially inert under the electrolysis conditions.
Platinized anodes are a recent innovation in the cathodic protection industry. These anodes employ an extremely thin film of platinum, on the order of 10 microns thick, over valve metal substrates, such as titanium, niobium and tantalum. When immersed in water, these valve metals are passivated and form an insulating film that does not break down at the normal operating voltages encountered in cathodic protection. The thin layer of platinum that is deposited on the substrate metal stops the formation of the insulating film on the valve metal and allows current to flow. If the platinum layer is scratched, the freshly exposed substrate metal passivates and stops passing current from the scratched area, but nevertheless continues to pass current from the rest of the platinum-coated substrate area. The consumption rate of platinum is on the order of 5-6 milligrams per ampere year. The high cost of platinum makes the platinized anodes expensive and that is why platinum is used in extremely thin layers. The thinness of these layers makes these anodes susceptible to abrasion damage and erosion-corrosion damage.
As is pointed out in British patent application No. 2,018,290 A, published Oct. 17, 1979, partly inert materials such as lead alloys or silicon-iron, and materials such as scrap iron or aluminum, are in fact attacked or take an active part in the cathodic electroprotection process. Electron-conducting non-metallic materials such as graphite and magnetite, Fe.sub.3 O.sub.4, are also used as anode materials for certain applications.
According to the British patent publication, an anode that is to be used for cathodic electroprotection ideally should be completely inert, even at high current densities; it should not polarize significantly; it should have a high electrical conductivity; it should be mechanically stable, and it should be economical. Stating the requirements for a good anode material somewhat difficulty than the British patent publication, the material for the anode should have essentially metallic conductivity, that is, the electrical conductivity that characterizes the more common metals, and in addition, it should have a low dissolution rate. The British patent publication sought to meet these requirements by forming an anode from a composite of magnetite with either lead or a lead alloy. In this composite, small particles of magnetite were dispersed in a matrix of the lead or the lead alloy.
Certain ceramic materials that are electron-conducting, such as the ferrites, exhibit dissolution or consumption rates that re many times less than those of the currently used silicon-iron and graphite materials often used in anodes. The ferrites have not gone into general use for this purpose in the past, howver, because ceramics are extremely brittle and cannot be fabricated readily.
Conducting ceramic anode coatings must provide an effective barrier to oxygen ions, so that the substrate metal does not become oxidized. In addition, the ceramic coating must have a relatively high electron conductivity. The coating must have an active surface area for oxidation to occur. The ceramic coating must also be mechanically strong and have good adherence to the substrate.
A past attempt to use magnetite as an anode material, in the form of a coating over a titanium or tantalum substrate, is described in U.S. Pat. No. 3,850,701. The magnetite was formed into a layer by a chemical process, with a thickness in the range from about 3 .mu.m to about 20 .mu.m. The magnetite layer was formed by electrodepositing iron onto a substrate of titanium, zirconium, tantalum, or niobium, and then applying a chemical treatment to the deposited iron to convert it to magnetite. This is not considered to be completely satisfactory, because of insufficient coating thickness and adhesion.
A Japanese publication by the authors, T. Fujii, T. Kodama, H. Baba, and S. Kitahara, "Anodic Behaviour of Ferrite-Coated Titanium Electrodes, Boshoku Gijutsu" (Corrosion Engineering), Vol. 29, 180-184 (1980), describes a different technique for the production of magnetite electrodes for use as insoluble anodes. The authors used a plasma jet spray technique for applying coatings of several spinel ferrites on titanium substrates for the production of insoluble anodes for cathodic protection. The coatings applied were up to about 50 .mu.m thick. The behavior of these anodes were measured in sodium chlorode solutions. The magnetite anode was said to show the lowest polarization but a higher dissolution rate than other ferrites. A tantalum coating over the titanium substrate was said to generate improved adhesion between the ferrite coating layer and the substrate. The ceramic coatings were too thin and did not have enough adhesion to produce a durable anode.